Integrated heterostructure of Group II-VI semiconductor materials including epitaxial ohmic contact and method of fabricating same

ABSTRACT

An ohmic contact to a p-type zinc selenide (ZnSe) layer in a Group II-VI semiconductor device, includes a zinc telluride selenide (ZnTe x  Se 1-x ) layer on the zinc selenide layer, a mercury selenide (HgSe) layer on the zinc telluride selenide layer and a conductor (such as metal) layer on the mercury selenide layer. The zinc telluride selenide and mercury selenide layers between the p-type zinc selenide and the conductor layer provide an ohmic contact by eliminating the band offset between the wide bandgap zinc selenide and the conductor. Step graded, linear graded, and parabolic graded layers of zinc telluride selenide may be provided. An integrated heterostructure is formed by epitaxially depositing the ohmic contact on the Group II-VI device. A removable overcoat layer may be formed on the Group II-VI device to allow room temperature atmospheric pressure transfer of the device from a zinc based deposition chamber to a mercury based deposition chamber, for deposition of the ohmic contact. A large area emitter may be formed by limiting the thickness of the mercury selenide layer so that optical radiation passes therethrough. A high efficiency optical emitter may be provided by using zinc telluride selenide or zinc sulfur telluride selenide to form an isoelectronic trap which produces broad and intense light output in the blue/green region.

U.S. GOVERNMENT RIGHTS

This invention was made with government support under Grant No.DMR-88-13525 awarded by the National Science Foundation. The U.S.Government has certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of application Ser. No.07/881,599 filed May 12, 1992.

FIELD OF THE INVENTION

This invention relates to semiconductor devices, and more particularlyto semiconductor devices formed of Group II-VI compound semiconductormaterials.

BACKGROUND OF THE INVENTION

Microelectronic applications of Group II-VI semiconductor materials havebeen widely investigated. In particular, the wide bandgap Group II-VIsemiconductor zinc selenide (ZnSe), and its related alloys such as zinccadmium selenide (ZnCdSe) and zinc sulfur selenide (ZnSSe), are beingwidely investigated as optoelectronic devices which are effective fromthe blue to the green region of the visible spectrum. As is well knownto those having skill in the art, the wide bandgap of these Group II-VIdevices compared to their equivalent nearly lattice matched Group III-Vor elemental semiconductor materials, make these wide bandgap GroupII-VI semiconductors potential candidates for blue to greenoptoelectronic devices. Blue to green optoelectronic sources arecurrently sought for a number of applications, including full colorelectroluminescent displays, read-write laser sources for high densityinformation storage on magnetic and optical media, sources for underseaoptical communications and other applications.

Molecular beam epitaxy and other fabrication techniques have recentlybeen developed so that both n-type and p-type doped layers of zincselenide and other related II-VI semiconductor materials may be grown.See for example a publication by Ren et al. entitled SubstitutionalDoping of ZnSe Films, Journal of Crystal Growth, Vol. 111, pp. 772-775,1991. It has also long been known to make Schottky contacts to n-typezinc selenide using mercury selenide (HgSe). See the publicationsentitled e, a Highly Electronegative Stable Metallic Contact forSemiconductor Devices by Best et al., Applied Physics Letters, Vol. 29,No. 7, pp. 433-434, 1976; Highly Electronegative Contacts to CompoundSemiconductors by Scranton et al., Journal of Vacuum Science andTechnology, Vol. 14, No. 4, pp. 930-934, 1977; and Lattice-MatchedHeterostructures as Schottky Barriers: HgSe/CdSe by Best et al., Journalof Vacuum Science Technology, Vol. 16, No. 5, pp. 1130-1133, 1979.

As a result of these and other developments, at least four researchgroups have recently described the fabrication of blue and/or greenlight emitting diodes and/or laser diodes from Group II-VIsemiconductors.

The first group is from North Carolina State University (NCSU) andincludes the present inventor. The fabrication of blue and green lightemitting diodes based on ZnSe and alloys thereof is described in apublication entitled ZnSe Light-Emitting Diodes by Ren et al., AppliedPhysics Letters, Vol. 57, No. 18, pp. 1901-1903, October, 1990, and Blue(ZnSe) and Green (ZnSe₀.9 Te₀.1) Light Emitting Diodes by Ren et al.,Journal of Crystal Growth, Vol. 111, pp. 829-832, 1991.

A second group of researchers from Brown University and PurdueUniversity have also described zinc selenide based laser diodes andlight emitting diodes in publications entitled Blue-Green InjectionLaser Diodes in (Zn,Cd)Se/ZnSe Quantum Wells by Jeon et al., AppliedPhysics Letters Vol. 59, No. 27, pp. 3619-3621, December, 1991;Blue/Green pn Junction Electroluminescence from ZnSe-based MultipleQuantum-Well Structures by Xie et al., Applied Physics Letters Vol. 60,No. 4, pp. 463-465, January, 1992; ZnSe Based Multilayer pn Junctions asEfficient Light Emitting Diodes for Display Applications, Jeon et al.,Applied Physics Letters, Vol. 60, No. 7, pp. 892-894, February, 1992;Blue and Green Diode Lasers in ZnSe-based Quantum Wells, Jeon et al.,Applied Physics Letters, Vol. 60, No. 17, April, 1992; and RoomTemperature Blue Light Emitting P-N Diodes from Zn(S,Se)-Based MultipleQuantum Well Structures, Xie et al., Applied Physics Letters, Vol. 60,No. 16, April, 1992, pp. 1999-2001.

A third group from 3M Company described a zinc selenide based laserdiode in an article entitled Blue-Green Laser Diodes by Haase et al.,Applied Physics Letters, Vol. 59, No. 11, pp. 1272-1274, September,1991. A fourth group from the University of Florida and Bellcoredescribed fabrication of LEDs and light emitting diodes using zincselenide in an article entitled Noncontact Electrical Characterizationof Low-Resistivity p-type ZnSe:N Grown by Molecular Beam Epitaxy by Parket al., Applied Physics Letters, Vol. 59, No. 15, pp. 1896-1898, 1991.

The above publications indicate that the art has now successfullyfabricated blue and green optoelectrical devices from ZnSe-basedmaterials. As Group II-VI fabrication processes are further refined,optical characteristics such as frequency spectrum width and operationallifetime are expected to improve due to reduced dislocation densities inthe materials and other improvements.

A major problem with all of these devices, however, has been the ohmic(nonrectifying) contact to zinc selenide, and in particular to p-typezinc selenide. This is a fundamental problem, which is related to thevery deep energy of the valence band of zinc selenide. As a consequence,contacts to p-type zinc selenide and related alloys, using conventionalmetals such as silver or gold, are not ohmic. In effect, the contactswhich have been considered by researchers as being ohmic, actuallyconstitute a reverse biased Schottky diode in series with the device,resulting in a large voltage drop across the supposedly ohmic contact.This large voltage drop results in almost all of the input power to thedevice being lost as heat. High voltages, of 20-50V or more, have beenrequired in order to induce optical emission, and the resultant heatdestroys the devices.

The "ohmic contact problem" for zinc selenide optical emitter deviceshas been widely reported. See for example the above cited 1990 Ren etal. article from the NCSU group, at page 1901: "It was not possible forus to complete Hall effect studies on the ZnSe:Li samples because ofproblems associated with non-ohmic contacts." See also the above cited1992 article by Xie et al. from the Brown-Purdue group, at page 463:"Hall measurements on the p-type layers were unreliable due to thedifficulty in forming ohmic contacts to the widegap semiconductor." Seealso the Haase et al. article by the 3M group at page 1273: "Heating inthese samples is a serious problem as the contact between the Au and thep-ZnSe presents a large barrier." Finally, see the Park et al. articlefrom the University of Florida/Bellcore group which notes that ". . .serious problems currently exist with regard to providing low-resistanceohmic contact to p-type ZnSe material . . . "

The above survey indicates that although significant advances have beenmade in fabricating Group II-VI devices, and in particular zinc selenidebased optoelectronic devices, the ohmic contact to these devices, and inparticular to p-type zinc selenide, remains a fundamental concern thathas heretofore eluded multiple independent groups of researchers.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved Group II-VI semiconductor device including an ohmic contacttherefor.

It is another object of the invention to provide an improved Group II-VIoptoelectronic device including an ohmic contact therefor.

It is still another object of the invention to provide an ohmic contactfor p-type zinc selenide and alloys thereof.

These and other objects are provided, according to the present inventionby an ohmic contact for a semiconductor device formed of Group II-VIsemiconductor materials, wherein the contact includes a lattice matchedmonocrystalline ternary compound Group II-VI semiconductor material onthe Group II-VI semiconductor device, a lattice matched monocrystallineGroup II-VI semimetal on the Group II-VI semiconductor material and aconductor, such as a metal, on the Group II-VI semimetal. The ternarycompound Group II-VI semiconductor material and the semimetal, betweenthe Group II-VI semiconductor material and the conductor layer,eliminate the band offset between the wide bandgap Group II-VIsemiconductor material and the conductor layer, to thereby provide anohmic contact for the Group II-VI semiconductor material.

In particular, according to the invention, an ohmic contact for zincselenide (ZnSe) or an alloy thereof, and particularly for p-type zincselenide or an alloy thereof, is provided by a heterostructure includinga zinc mercury selenide (Zn_(x) Hg_(1-x) Se) layer or a zinc tellurideselenide (ZnTe_(x) Se_(1-x)) layer on the layer of zinc selenide oralloy thereof, and a mercury selenide (HgSe) layer on the zinc mercuryselenide or zinc telluride selenide layer. A conductor (such as metal)layer is formed on the mercury selenide layer. The zinc mercury selenideor zinc telluride selenide layer, and the mercury selenide layer betweenthe p-type zinc selenide and the conductor layer, provide an ohmiccontact by eliminating the band offset between the wide bandgap zincselenide and the conductor.

Many alternative formulations of the zinc mercury selenide layer and thezinc telluride selenide layer of the present invention may be provided.In particular, the ratio of zinc to mercury, or tellurium to selenium,may be constant across the layer to provide a step graded zinc mercuryselenide layer or zinc telluride selenide layer. Alternatively, theamount of mercury or tellurium may increase from the zinc selenide layerto the mercury selenide layer. One form of increase is a linearincrease, to provide a linear graded zinc mercury selenide or zinctelluride selenide layer. Alternatively, and preferably, the amount ofmercury or tellurium increases nonlinearly from the zinc selenide layerto the mercury selenide layer to provide a nonlinear graded zinc mercuryselenide or zinc telluride selenide layer. Preferably, a parabolicgraded layer is provided, in which the amount of mercury or telluriumincreases parabolically.

According to the invention, at least some of the zinc mercury selenidelayer or zinc telluride selenide layer adjacent the p-type zinc selenidelayer is doped p-type. Preferably, half the thickness of the zincmercury selenide layer or zinc telluride selenide layer, adjacent thezinc selenide layer, is doped p-type, and the other half of the zincmercury selenide layer or zinc telluride selenide layer, adjacent themercury selenide layer, is of neutral conductivity. Modulation dopingmay also be used in the parabolically graded zinc mercury selenide layeror zinc telluride selenide layer.

It has also been found, according to the present invention, that ap-type layer of mercury selenide between the p-type zinc selenide layerand the conductor layer, can provide an ohmic contact for p-type zincselenide which is far superior to conventional gold or silver ohmiccontacts. The ohmic contacts of the present invention produce a nearlyideal (linear) voltage-current relation, indicating that the contactsare indeed ohmic. Hall effect measurements may also be taken, whichverify that the contacts are ohmic. When a Group II-VI optoelectronicdevice is integrated with the ohmic contacts of the present invention,an integrated heterostructure device is formed, with one heterostructureproviding an optical function such as e.g. light emission, and anotherheterostructure providing an electrical function such as ohmic contact.These integrated heterostructures can emit radiation in the blue togreen frequency range with an input voltage of five volts or less,resulting in high efficiency Group II-VI optoelectronic devices.

According to another aspect of the present invention, in certainapplications it is desirable to provide large area optical emitters suchas light emitting diodes (LED), in which the light is emitted from thetop and sides of the LED. Such large area LEDs can be used to producenumerical display arrays and many other devices. According to theinvention, the integrated heterostructures of the present invention maybe used to produce large area optical emitters by providing a mercuryselenide layer, or a mercury selenide layer on the zinc mercury selenideor zinc telluride selenide layer, which is sufficiently thick to providean ohmic contact to the p-type zinc selenide layer, and is sufficientlythin to pass optical emissions from the optical emission heterostructuretherethrough. Preferably, a mercury selenide layer less than about 100 Åthick, is provided. Although mercury selenide is a semimetal whichabsorbs radiation through the visible and infrared spectral regions, thethin mercury selenide layer is thin enough to limit the light absorptionlosses to less than 10%. Moreover, since mercury selenide is of veryhigh conductivity, the mercury selenide layer is sufficiently thick toact as a semitransparent electrode which covers the top of theheterostructure. In order to further reduce the surface contactresistance, a transparent conductor layer such as indium tin oxide maybe formed on the thin layer of mercury selenide.

An integrated heterostructure according to the present invention may beformed by epitaxially forming an optical heterostructure and anelectrical heterostructure as described above. Typically, the opticalheterostructure (such as a laser diode or light emitting diode) isformed in a first chamber, using molecular beam epitaxy (MBE) of zincselenide and related alloys. The zinc mercury selenide electricalheterostructure (ohmic contact) is typically fabricated in a seconddeposition chamber because of the special requirements for depositingmercury based materials. Transfer between the first and second chamberstypically must take place under ultra high vacuum conditions.

Alternatively, according to the present invention, a mercury basedelectrical heterostructure may be formed on a zinc selenide basedoptical heterostructure without the need for ultra high vacuum transferbetween MBE chambers, by forming a thin overcoat layer, for example,selenium, on the optical heterostructure. This structure can then beremoved from the zinc selenide MBE chamber and inserted into a mercurybased MBE chamber at room temperature and under atmospheric conditions.Once the structure is placed in the mercury based chamber, and thechamber is evacuated, the selenium overcoat layer may be evaporated andthe mercury based electrical heterostructure may be epitaxially formedon the zinc selenide based optical heterostructure.

Similarly, in order to prepare the integrated heterostructure of thepresent invention which includes a zinc telluride selenide layer, theoptical heterostructure (such as a laser diode or light emitting diode)is formed in a first chamber, using molecular beam epitaxy (MBE) of zincselenide and related alloys. The zinc telluride selenide layer of theelectrical heterostructure (ohmic contact) is also fabricated in thefirst deposition chamber, for example by continuing molecular beamepitaxy growth at about 250° C. Growth of a graded zinc tellurideselenide layer may be accomplished by decreasing the temperature of theselenium MBE source oven, preferably under computer control, whileincreasing the source temperature of the tellurium MBE source oven. Anoptional thin layer (about 100 Å) of p-type zinc telluride may be formedon the graded layer of zinc telluride selenide.

Following deposition of the zinc telluride selenide graded layer, a thinovercoat layer, for example selenium, is formed. The structure can thenbe removed from the first MBE chamber and inserted into a second,mercury based MBE chamber at room temperature and under atmosphericconditions. Once the structure is placed in the mercury based chamber,and the chamber is evacuated, the selenium overcoat layer may beevaporated, and a layer of mercury selenide may be deposited at about100° C. in the second MBE chamber. Accordingly, the zinc tellurideselenide layer can be grown in a first MBE chamber using the same lowgrowth temperature (about 250° C.) as the optical heterostructure of theintegrated heterostructure device. The graded layer can be doped p-typeusing nitrogen from the same remote plasma source that is used toprepare the p-type layers of the optical portion of the integratedheterostructure device. Accordingly, growth of the layer of zinctelluride selenide is simpler than the growth of the zinc mercuryselenide layer of application Ser. No. 07/881,599.

According to yet another aspect of the present invention, a highefficiency blue/green optical emitter is formed using a layer of thequaternary alloy zinc sulfur telluride selenide (ZnS_(x) Te_(y) Se_(z),where x+y +z=1) between two layers of zinc sulfur selenide (ZnSSe), or alayer of the ternary compound zinc telluride selenide (ZnTe_(x)Se_(1-x)) between layers of zinc selenide (ZnSe). The zinc sulfurtelluride selenide or zinc telluride selenide layers form a quantumwell, and multiple layers may be used to form multiple quantum wells. Inan LED application, a layer about 1000 Å thick will typically be formedwhile in a laser, a layer about 100-200 Å thick will be formed.Tellurium forms an isoelectronic trap which produces broad and intenselight output in the blue/green spectral region.

The cladded zinc sulfur telluride selenide or zinc telluride selenidestructure is preferably formed on a substrate such as gallium arsenideor zinc selenide. Zinc sulfur selenide and zinc sulfur tellurideselenide ar used with gallium selenide substrates because of their closelattice match thereto. For zinc selenide substrates, zinc selenide andzinc telluride selenide are used. A mercury selenide, zinc mercuryselenide or zinc telluride selenide layer is used to form an ohmiccontact to the zinc selenide or zinc sulfur selenide. The emitters arerelatively easy to fabricate because no adjustments need be made to thezinc, sulfur, tellurium or selenium fluxes. Rather, these fluxes needonly be turned on and off at the appropriate time during deposition toform the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a first embodiment of anoptical emitter having an ohmic contact according to the presentinvention.

FIG. 2 graphically illustrates energy bandgap versus lattice constantfor selected semiconductor materials.

FIG. 3 is an energy band diagram illustrating the band offsets betweenzinc selenide and gallium arsenide.

FIG. 4 is an energy band diagram showing the energy band lineup betweengold and zinc selenide.

FIG. 5 is an energy band diagram illustrating the band lineups of a zincselenide p-on-n diode under forward bias.

FIG. 6 graphically illustrates the current-voltage characteristics ofvarious zinc selenide based heterostructures.

FIG. 7 is an energy band diagram showing the band lineup between cadmiumtelluride and mercury telluride.

FIG. 8 is an energy band diagram showing the band lineups between zincselenide and mercury selenide.

FIG. 9 is a graphical illustration of the voltage current relationshipfor an ohmic contact according to the present invention.

FIG. 10a illustrates the current-voltage relationship for a known zincselenide light emitting diode.

FIGS. 10b-10d illustrate the current-voltage relationships for lightemitting diodes including an ohmic contact according to the presentinvention.

FIG. 11 is a graphical illustration of the current-voltage relationshipamong known light emitting diodes and light emitting diodes fabricatedaccording to the present invention.

FIG. 12 is a cross-sectional illustration of a second embodiment of thepresent invention.

FIG. 13 is an energy band diagram for the device shown in FIG. 12,including a step graded zinc mercury selenide layer.

FIG. 14 is an energy band diagram for the device shown in FIG. 12,including a linear graded zinc mercury selenide layer.

FIG. 15 is an energy band diagram for the device shown in FIG. 12,including a parabolic graded zinc mercury selenide layer.

FIGS. 16a-16p are cross-sectional illustrations of alternativecombinations of integrated heterostructures according to the presentinvention.

FIG. 17 illustrates another integrated heterostructure according to thepresent invention.

FIGS. 18a and 18b illustrate steps for forming the integratedheterostructure of FIG. 17 according to the present invention.

FIGS. 19a and 19b illustrate alternative steps for forming theintegrated heterostructure of FIG. 17 according to the presentinvention.

FIG. 20 graphically illustrates the current-voltage relationship for alight emitting diode which does not include an ohmic contact accordingto the present invention.

FIG. 21 graphically illustrates the current-voltage relationship for alight emitting diode which includes an ohmic contact according to thepresent invention.

FIGS. 22 and 23 are cross-sectional illustrations of alternativeintegrated heterostructures according to the present invention.

FIG. 24 graphically illustrates wavelength versus intensity for thestructure of FIG. 22.

DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which a preferred embodimentof the invention is shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiment set forth herein; rather, this embodiment is provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of the invention to those skilled in the art. Like numbers referto like elements throughout.

Referring now to FIG. 1, a first embodiment of an optical emitter havingan ohmic contact according to the present invention is illustrated.Optical emitter 10 is an integrated heterostructure including an opticalheterostructure 11 and an electrical heterostructure 12. Opticalheterostructure can be a well known blue to green light emitting diodeor laser diode structure, or any other Group II-VI device now known ordiscovered later. As shown in FIG. 1, the optical heterostructure i is ablue light emitting diode (LED). The blue LED is formed on a substrate15 of zinc selenide, gallium arsenide or any other known material. TheLED is a p-n junction between n-type zinc selenide layer 16 and p-typezinc selenide layer 17. Both zinc selenide layers may be formed onsubstrate 15 using well known techniques such as molecular beam epitaxy,and both layers may be doped at 10¹⁸ carrier per cubic centimeter Bottomcontact 14 is formed of metal such as gold-germanium for n-type GaAs orindium for n-type ZnSe, or other well known conductors. It will beunderstood by those having skill in the art, that, as used herein, whena layer is formed "on" another layer, it may be formed directly on theunderlying layer, or one or more intervening layers may be present.

Still referring to FIG. 1, the electrical portion 12 of the integratedheterostructure 10 comprises a layer of mercury selenide. As is wellknown to those having skill in the art, mercury selenide is a semimetalrather than a semiconductor, because it has a bandgap that is zero. Itwill be understood by those having skill in the art that the integratedoptical structure 11 and electrical structure 12 is epitaxially formedas a monolithic structure, with lattice match between adjacent layers soas to produce minimal dislocations at the junctions between the layers.FIG. 2 illustrates the energy bandgap versus lattice constant forvarious Group IV single element semiconductors, Group III-VI compoundsemiconductors and Group II-VI compound semiconductors and semimetals.

It has been found, according to the invention, that mercury selenideforms a good ohmic contact to p-type zinc selenide and thereby solvesthe last fundamental problem in development of high efficiency GroupII-VI devices. FIG. 3 illustrates the origin of the ohmic contactproblem. As shown in FIG. 3, there is essentially no conduction band(E_(c)) offset between zinc selenide and gallium arsenide. As aconsequence, valence band (E_(v)) offset is very large: about 1.3electron volts (eV). The deep valence band of zinc selenide is thesource of the ohmic contact problem. Gold, which has the largest workfunction of all metals, has proven to be an ineffective ohmic contact tozinc selenide. Gold forms a barrier of about 1.1 electron volts onn-type zinc selenide. A gold/zinc selenide interface, as shown in FIG.4, produces a band offset of about 1.6 electron volts. Accordingly, theuse of gold as an ohmic contact to a p-on-n zinc selenide homojunction,produces the band bending shown in FIG. 5.

Under forward bias of the zinc selenide diode, the gold/p-type zincselenide junction is reverse biased and current flow is limited bycarrier tunneling through the heterobarrier. This is illustrated by thecurrent-voltage characteristics shown in FIG. 6 as reported by the 3MGroup for their initial laser diode. Data for LED's prepared by the NCSUgroup is also shown to illustrate that the forward characteristics aresimilar for samples prepared in different laboratories. This datastrongly suggests that the current limiting device is a series diodeundergoing reverse breakdown. This diode is associated with reversebreakdown via carrier tunneling of the gold-zinc selenide Schottky diodeheterointerface.

In order to develop high efficiency LEDs and lasers from zinc selenideand related materials, the ohmic contact problem must be solved. Asolution to the ohmic contact problem has been found by studying thevalence band offset between mercury telluride (HgTe) and cadmiumtelluride (CdTe). As shown in FIG. 7, it has been found that the valenceband offset is 0.35 electron volts. Earlier estimates, based on the"common anion rule" implied a band offset of zero. Accordingly, thecommon anion rule must be rephrased as a "modified common anion rule"for Group II-VI materials containing mercury which is consistent withthe mercury telluride/cadmium telluride valence band offset results. Asshown in FIG. 7, a modified common anion rule gives:

    ΔE.sub.c /ΔE.sub.v =1.25/0.350=3.6             (1)

If Equation (1) holds for the mercury selenide/zinc selenide interface,then

    ΔE.sub.c +ΔE.sub.v =Eq.sub.(ZnSe) =2.70 eV     (2)

and a valence band offset ΔE_(v) of 0.59 electron volts is obtained forthe mercury selenide/zinc selenide interface as shown in FIG. 8. Thisrepresents nearly a threefold decrease compared to a gold/zinc selenideinterface barrier of about 1.6 electron volts. Thus, a layer ofepitaxial mercury selenide on p-type zinc selenide should provide a muchbetter ohmic contact than gold.

FIG. 9 graphically illustrates the voltage-current relationship for anohmic contact according to the present invention. In order to fabricatethis ohmic contact, a layer of mercury selenide was grown, by molecularbeam epitaxy, onto p-type (nitrogen doped) zinc selenide samples.Standard photolithographic processing was then used to define fourcontacts on each of the samples for Hall effect measurements. FIG. 9shows the current-voltage relationship between adjacent Hall contactsone centimeter apart of a representative sample. Apart from a small(about 0.1 volt) offset, the contacts are very nearly ohmic, thusindicating that electrical contacts, good enough for Hall effectmeasurement, are now available.

Several diode structures were prepared using molecular beam epitaxydeposition. The p-type layers included zinc sulfur selenide doped withnitrogen followed by a layer of nitrogen doped p-type zinc selenide anda layer of mercury selenide. Diodes were then prepared for testing usingstandard techniques.

FIG. 10a illustrates the current-voltage relationship of an earlierdevice which used lithium as the p-type dopant and gold as theelectrical contact to the p-type zinc selenide layer. As shown, about 10V is required to obtain 1 mA current under forward bias conditions for adiode having a cross sectional area of 10⁻³ cm². FIGS. 10b-10dillustrate diodes including zinc selenide and mercury selenide layers,according to the present invention. As shown, these diodes ofcross-sectional area 10⁻³ cm² turn on at a much lower voltage (2.1 V toobtain 1 mA, 3.2 V to obtain 10 mA and 4.4 V to obtain 50 mA as shown inFIGS. 10b-10d, respectively). This dramatic increase in current at agiven voltage compared with earlier diodes provides clear evidence thatthe ohmic contact of the present invention is effective.

FIG. 11 graphically compares the diodes fabricated according to thepresent invention as described above ("NCSU p-on-n diodes 2/92") withthose produced by 3M and Purdue. As shown, the p-on-n diodes of thepresent invention are substantially improved from those reported by 3M.Indeed, the diodes of the present invention are substantially betterthan the Purdue results for n-on-p diodes, particularly when diodes ofsimilar cross-sectional areas are compared.

Referring now to FIG. 12, a second integrated heterostructure 20 isshown which includes a further improved ohmic contact from that ofFIG. 1. As shown in FIG. 12, the optical heterostructure 11 is identicalto that of FIG. 1. However, the electrical heterostructure comprises alayer of zinc mercury selenide (Zn_(x) Hg_(1-x) Se) 19 or Zinc tellurideselenide (ZnTe_(x) Se_(1-x)) between the layer of mercury selenide 18and the p-type zinc selenide layer 17. The layer of mercury selenide 18and the layer of zinc mercury selenide or zinc telluride selenide 19provide an improved ohmic contact between p-type zinc selenide layer 17and conductor 18.

According to the invention, alternative embodiments of zinc mercuryselenide layer or zinc telluride selenide layer 19 can provide an ohmiccontact. In one embodiment, zinc mercury selenide layer or zinctelluride selenide layer 19 is a step graded layer having uniform ratioof zinc to mercury or tellurium to selenium respectively, across theentire thickness thereof. A preferred uniform ratio for this layer isabout 1:1. FIG. 13 illustrates the energy band diagram for such a stepgraded zinc mercury selenide layer. Preferably the step graded layer ofzinc mercury selenide or zinc telluride selenide is doped p-type withthe same doping concentration (e.g. 10¹⁸) as the zinc selenide layer.Nitrogen doping is preferred.

Another alternative is shown in FIG. 14 wherein the amount of mercury ortellurium increases linearly across the zinc mercury selenide or zinctelluride selenide layer respectively, from the zinc selenide layer tothe mercury selenide layer, to thereby provide a linear graded zincmercury selenide or zinc telluride selenide layer. Preferably, thelinear graded layer of zinc mercury selenide or zinc telluride selenideis doped p-type with the same doping concentration as the p-type zincselenide (e.g. 10¹⁸).

Referring now to FIG. 15, the most preferred embodiment presentlycontemplated by the inventor, is a parabolic graded zinc mercuryselenide or zinc telluride selenide layer. As further shown in FIG. 15,at least part of the zinc mercury selenide layer or zinc tellurideselenide layer is doped p-type, and preferably a portion of the zincmercury selenide layer or zinc telluride selenide adjacent the zincselenide layer is doped p-type. Most preferred is a configurationwherein half the thickness of the zinc mercury selenide layer or zinctelluride selenide layer adjacent the zinc selenide layer is dopedp-type, with the same doping concentration as the zinc selenide, andwith half the layer adjacent the mercury selenide being undoped. In oneconfiguration, the zinc mercury selenide layer or zinc tellurideselenide layer is 1200 Å thick, with 600 Å adjacent the zinc selenidelayer being doped p-type (preferably with nitrogen) at 10¹⁸ carriers percubic centimeter to match that of the p-type zinc selenide, and with 600Å adjacent the mercury selenide being undoped. An optional thin (about100 Å) layer of p-type zinc telluride (ZnTe) may be formed on the gradedzinc telluride selenide layer, to ensure that the interface with themercury selenide layer is pure ZnTe.

It will be understood by those having skill in the art that in displayand other applications it is desirable to provide broad area opticalemitters which preferably emit light from the entire top and sides ofthe emitter. According to the invention, the thickness of mercuryselenide layer is limited so that the mercury selenide layer acts as asemitransparent electrode, with a substantial portion of the lightemitted from the optical emitter passing through the layer of mercuryselenide 18. Preferably, the layer of mercury selenide is less thanabout 100 Å thick. In this case, although mercury selenide is asemimetal which absorbs throughout the visible and infrared spectralregions, the mercury selenide layer is thin enough so that the lightabsorption losses are less than about 10%. In addition, the very highconductivity of the mercury selenide layer allows it to act as asemitransparent electrode which covers the entire top of the opticalstructure. Blue light emitting diodes which emit light at 476 nm over alarge surface area with a room temperature efficiency of greater than0.02% may be obtained using operational voltages of only 4.5 volts toproduce 50 mA through the integrated heterostructure.

In order to further reduce the resistance of the large area emitter,conductor 13 may be formed of indium tin oxide, a well known transparentelectrode material. The use of indium tin oxide conductor 13 incombination with a mercury selenide layer 18, less than 100 Å thick,enhances the current spreading at the top of the emitter so thatnumerals or other symbols approaching 1 cm by 1 cm in surface area canbe fabricated.

It will also be understood by those having skill in the art that any ofthe ohmic contacts 12 of the present invention may be combined with anyGroup II-VI devices to form an integrated heterostructure device. FIGS.16a-16p illustrate various combinations.

FIGS. 16a-16d illustrate green LEDs. In particular, FIG. 16a illustratesa mesa diode green LED including a ZnCdSe quantum well 31. FIG. 16billustrates a multiple quantum well (MQW) mesa diode green LED includingan MQW 32 of ZnSe and ZnCdSe. FIG. 16c illustrates a GRaded IndexSeparate Confinement Heterostructure ("GRINSCH") mesa diode green LEDincluding a layer of graded p-type ZnCdSe 33, a ZnCdSe quantum well 34and graded n-type ZnCdSe 35. FIG. 16d illustrates a GRINSCH-MQW mesadiode green LED including a graded layer of p-type ZnCdSe 36 and a layerof ZnCdSe 37, and a graded layer of n-type ZnCdSe 38.

FIGS. 16e-16h illustrate blue LEDs. FIG. 16e illustrates a mesa diodeblue LED including p- and n-type layers of zinc sulfur selenide and azinc cadmium selenide quantum well 4. FIG. 16f illustrates an MQW mesadiode blue LED including an MQW 42 of zinc sulfur selenide and zinccadmium selenide. FIG. 16g illustrates a GRINSCH mesa diode blue LEDincluding a layer graded p-type zinc sulfur selenide layer 43, a zinccadmium selenide quantum well 44 and a graded n-type zinc sulfurselenide layer 45. FIG. 16h illustrates a GRINSCH MQW blue LED includinga graded layer of p-type zinc sulfur selenide 46, an MQW of zinc sulfurselenide and zinc cadmium selenide 47, and a graded layer of n-type zincsulfur selenide 48.

FIGS. 16i-16l illustrate green lasers. FIG. 15i illustrates a GRINSCHgreen laser, including a graded layer of p-type zinc cadmium selenide51, a zinc cadmium selenide quantum well 52, and a graded layer ofn-type zinc cadmium selenide 53. FIG. 16j illustrates a separateconfinement green laser formed by a layer of p-type zinc cadmiumselenide 54, a zinc cadmium selenide quantum well 56 and an n-type zinccadmium selenide layer 57. FIG. 16k illustrates a GRINSCH MQW greenlaser including a graded layer of p-type zinc cadmium selenide 58, azinc cadmium selenide MQW 59 and a graded layer of n-type zinc cadmiumselenide 61. A separate confinement MQW green laser is shown in FIG. 16lincluding a p-type layer of zinc cadmium selenide 62, a zinc cadmiumselenide MQW 63 and n-type zinc cadmium selenide layer 64.

FIGS. 16m-16p illustrate blue lasers. In particular, FIG. 16millustrates a GRINSCH MQW blue laser formed of graded p-type zinc sulfurselenide layer 66, an MQW 67 of zinc sulfur selenide and zinc cadmiumselenide, and a graded layer of n-type zinc sulfur selenide 68. FIG. 16nillustrates a GRINSCH blue laser including graded p-type zinc sulfurselenide layer 71, a zinc cadmium selenide layer quantum well 72 and agraded n-type zinc sulfur selenide layer 73. FIG. 15o illustrates aseparate confinement blue laser including a p-type layer of zinc sulfurselenide 75, a zinc cadmium selenide quantum well 76 and an n-type zincsulfur selenide layer 77. Finally, FIG. 15p illustrates a separateconfinement MQW blue laser including a p-type layer of zinc sulfurselenide 81, an MQW of zinc sulfur selenide and zinc cadmium selenide82, and an n-type layer of zinc sulfur selenide 83.

Each of the structures of FIGS. 15a-15p includes the ohmic contact shownin FIG. 12, including a layer of mercury selenide 18 and a graded zincmercury selenide layer or zinc telluride selenide layer 19. The ohmiccontact of FIG. 1, which includes a mercury selenide layer 18 and isfree of a zinc mercury selenide layer 19, may also be used. It will beunderstood that other configurations of the optical structure 1 may alsobe used.

It will also be understood that in any of the heterojunctions previouslydescribed, band discontinuities can be reduced or eliminated bymodulation doping. Modulation doping is a technique which is well knownto those having skill in the art, and is described for Group III-Vsemiconductors in an article by Shupert et al. entitled Elimination ofHeterojunction Band Discontinuities By Modulation Doping, AppliedPhysics Letters, Vol. 16, No. 4, January 1992, pp. 466-468.

Techniques for fabricating the ohmic contacts of the present inventionwill now be described. At present, molecular beam epitaxy (MBE) is thepreferred deposition technique for preparing integrated heterostructuredevices of Group II-VI compound semiconductors for blue to green lightemitter applications. This is because p-type doping is possible usingnitrogen excited in a remote-plasma-enhanced MBE source, such as themodel MPD21 marketed by Oxford Applied Research, Oxfordshire, England.Organometallic vapor deposition (OMCVD) is a second potential depositiontechnique for growing integrated heterostructure devices, if p-typedoping is developed. At present, p-type doping of zinc selenide andrelated alloys by OMCVD has not been demonstrated, to the best of theinventor's knowledge.

The integrated heterostructure devices of the present invention may befabricated using a two chamber MBE deposition system. In the integratedheterostructure including the zinc mercury selenide layer 19, a firstdeposition chamber may be used exclusively for the deposition of widebandgap Group II-VI semiconductor layers including zinc selenide, zinccadmium selenide and zinc sulfur selenide, to grow the optical portion11 of the integrated heterostructure device according to any of thestructures described above or any other Group II-VI optical structure.The sample is then transferred to a second chamber where mercuryselenide, or zinc mercury selenide and mercury selenide, isheteroepitaxially formed on the p-type zinc selenide. A second chamberis typically required because of the special requirements for depositingmercury based materials which are necessitated by the extremely highvapor pressure of mercury (about 3 mTorr at room temperature).Accordingly, as is well known to those having skill in the art, specialMBE sources, cryoshrouding and other special techniques may need to beemployed.

A detailed process for forming a separate confinement green laser havingan ohmic contact using zinc mercury selenide according to the presentinvention will now be described. The laser structure is shown in FIG. 17and includes an optical heterostructure 11 comprising a zinc cadmiumselenide quantum well 87 between p- and n-type zinc selenide layers 86and 88, respectively, which are placed between p-and-n zinc sulfurselenide layers 85 and 89, respectively. The optical heterostructure 11is formed on substrate 15 and includes a top layer 17 of p-type zincselenide. The ohmic contact heterostructure comprises a layer of gradedzinc mercury selenide 19 and a layer of mercury selenide 18 betweenmetal electrode 13 and p-type zinc selenide layer 17.

Conventional substrate preparation processes are used to form ann+gallium arsenide or zinc selenide substrate 15. Epitaxial growth ofthe p-on-n heterostructure is initiated at a substrate temperature of220°-260° C. Chlorine, in the form of the solid zinc chloride, ispresently the preferred n-type dopant for the n-type zinc sulfurselenide layer 89 and the n-type zinc selenide optical confinement layer88. Doping levels for layers 88 and 89 should be about 10¹⁸ cm⁻³. Thezinc cadmium selenide quantum well 87 is then deposited, followed by thep-type zinc selenide layer 86 and the p-type zinc sulfur selenide layer85.

Two techniques may be used to effectively form layers 17, 18 and 19 onlayer 85. A first technique transfers the already completed structureinto a second chamber under ultrahigh vacuum, using commerciallyavailable ultrahigh vacuum interlocking systems or a portable ultrahighvacuum wafer transfer device. Alternatively, a second technique avoidsthe need to transfer the optical heterostructure to a second depositionchamber under ultrahigh vacuum by forming a thin overcoat layer, forexample selenium, on the structure.

FIGS. 18a and 18b illustrate the use of the selenium overcoat layer.Referring to FIG. 18a, a thin (e.g. about 0.1 μm) layer of selenium 91may be formed in the first chamber 92 after the deposition sequencedescribed above. The selenium layer deposition may take place at roomtemperature. Then, the sample can be removed from the first chamber 92and inserted into a second chamber 93 for mercury based heterostructuregrowth. In the second chamber 93, the selenium layer 91 may beevaporated by heating the substrate at 300° C. for several minutes justprior to mercury based film growth to obtain the structure of FIG. 18b.

Once the sample in chamber 93 is ready for deposition, a thin(approximately 200 Å) p-type layer of zinc selenide 17 is depositedfollowed by a graded p-type zinc mercury selenide region 19, and a thin(approximately 200 Å) layer of mercury selenide 18. The exact thicknessof the layers is not critical. Preferably, at least part or all of thegraded zinc mercury selenide layer is doped p-type to effectivelyeliminate band discontinuities. At present, nitrogen from a remoteplasma source is the preferred p-type dopant. During deposition of theZn_(x) Hg_(1-x) Se graded layer, in which the x value is graded from x=1to x=0, the substrate temperature is preferably reduced from about220°-260° C. initially to about 100° C. in order to effectivelyincorporate the mercury into the graded zinc mercury selenide layer 19.Zinc and selenium fluxes must also be adjusted for proper growth of thezinc mercury selenide layer. If this adjustment is not done, practicallyall of the mercury reevaporates and grading of the layer does not occur.The final mercury selenide layer 18 is then deposited at about 100° C.

FIGS. 19A and 19B illustrate a two chamber process using a seleniumovercoat layer to form an integrated heterostructure device including azinc telluride selenide layer 19. Referring to FIG. 19A, the opticalheterostructure 11 and p-type zinc selenide layer 17 are fabricated infirst chamber 92 as Was already described in FIG. 18A. Then, zinctelluride selenide layer 19 is grown in first chamber 92 by continuinggrowth at about 250° C. A graded layer of zinc telluride selenide thatis doped p-type with nitrogen from the plasma source is grown bydecreasing the temperature of the selenium MBE source oven undercomputer control while increasing the source temperature of thetellurium MBE source oven. In this way, a p-type graded layer whichinitially is composed of zinc selenide and finally is composed of zinctelluride is prepared. A preferred thickness of between about 500-1500 Åis grown. The final fluxes may be maintained for an additional shorttime period to form an optional thin layer (about 100 Å) of p-type zinctelluride. Additional growth of zinc telluride ensures a pure zinctelluride interface with the mercury selenide layer formed thereon.

Following deposition of the zinc tellurium selenide layer 19, a cappingformed as was already described in connection with FIG. 18A. Theselenium capped structure is then transferred to the second MBE chamber93 as shown in FIG. 19B. As was already described in connection withFIG. 18B, in the second chamber 93, the selenium layer 91 is desorbedand a thin layer of mercury selenide 18 (FIG. 17) is deposited at about100° C. in the mercury based MBE chamber.

The zinc telluride selenide layer 19 of the present invention can begrown in the first MBE chamber 92 using the same growth temperature(about 250° C.) as the optical heterostructure 11 of the integratedheterostructure device. Moreover, the graded layer can be doped p-typeusing nitrogen from the same doping source that is used to prepare thep-type layers of the optical portion 1 of the integrated heterostructuredevice. Accordingly, the fabrication process for the graded layer ofzinc telluride selenide is simpler than the graded zinc mercury selenidelayer described in FIGS. 18A and 18B.

FIGS. 20 and 21 graphically illustrate the improved performance obtainedusing the zinc telluride selenide layer 1 according to the presentinvention. Two identical structures, differing only in the presence ofthe zinc telluride selenide layer were grown to compare the performanceof the integrated heterostructure with and without the zinc tellurideselenide layer. In particular, the optical heterostructure 11 of FIG.16E was grown in the first MBE system 92 (FIG. 18A or 19A) and cappedwith a thin layer of selenium. The structure was then transferred to thesecond MBE system 93 (FIG. 18B or 19B) where the selenium was desorbedand the thin mercury selenide layer was grown at about 100° C. Then,half of the sample was processed into light emitting diodes, while theother half was reinserted into first chamber 92 (FIG. 18A or 19A). Themercury selenide layer was desorbed and a linear graded, all p-typelayer of zinc telluride selenide was grown. The sample 18 was againcapped with selenium and reinserted into the second MBE chamber 93 (FIG.19B) where the selenium layer was desorbed and the thin mercury selenidelayer was grown by MBE. Accordingly, a sample with identical opticalheterostructures was prepared, with and without the zinc tellurideselenide layer 19.

FIG. 20 illustrates the properties of the LED structure with the zinctellurium selenide layer 19 absent. As shown, the diode turns on, orbegins conducting appreciable current, at about 2.8 volts and generates10 milliamperes at about 4.4 volts. The forward bias characteristics are"soft", because of the 0.6 electron volt barrier between mercuryselenide and zinc selenide.

FIG. 21 graphically illustrates the properties of the identical LEDstructure including the layer of zinc telluride selenide. As shown, theturn-on voltage is reduced to about 1.9 electron volts and the LEDgenerates 10 milliamperes at about 3.2 electron volts. In addition, theforward bias characteristics of the diode correspond to a seriesresistance of only about 75 Ω. This is reflected in the much sharperforward bias characteristics compared with FIG. 20.

Hall effect measurements using a mercury selenide ohmic contact yield aroom temperature mobility m_(p) of 19 cm⁻² /V-s and a room temperaturehole concentration of 5×10¹⁸ cm⁻³. Based on a review of the literature,it is estimated that the valance band of zinc telluride is within 0.2eV±0.4 eV of that of mercury selenide. See the publications by R. H.Miles et al. entitled Superattices of II-VI Semiconductors, Journal ofCrystal Growth, Vol. 85, pp. 188-193 (1987), Kobayashi et al. entitledGrowth and Characterization of ZnSe-ZnTe Strane-Layer Superlattices,Journal of Crystal Growth, Vol. 81, pp. 495-500 (1987), and Y.Rajakarunanayake et al. entitled Ban Offset of the ZnSe-ZnTeSuperlattices: A Fit to Photoluminescence Data by k.p Theory, Journal ofVacuum Science Technology, Vol. B6, No. 4, July/August 1988, pp.1354-1359. By optimizing the p-type doping of the zinc tellurideselenide layer and its grading, shape and thickness, it should bepossible to further reduce the series resistance to less than 10 Ω.

Similar procedures may be used to fabricate a variety of otherintegrated heterostructures for blue laser, blue LED, green laser andgreen LED applications, and other applications. The color output of aparticular device is determined by the cadmium content of the zinccadmium selenide layers in the active regions of the structure. Thedegree of optical confinement is determined by the sulfur content in thezinc sulfur selenide cladding layers. The graded structure of p-typedoped zinc mercury selenide or zinc telluride selenide 19 furtherreduces the valance band offset between mercury selenide and zincselenide and thus further reduces the series resistance of the ohmiccontact. The series resistance and turn-on voltage of the optical deviceis thereby minimized.

It is also possible to grow p-on-n integrated heterostructure devices oninsulating substrates. This might be desirable, for example, because ofthe limited availability of n-type ZnSe substrates at present. If aninsulating substrate is employed, the multilayered growth sequenceremains unchanged from that described above. However, the finishedwafers are processed differently so that an ohmic contact can be made tothe initial n-type epitaxial layer of the structure. This can be done byemploying standard photolithography/etching techniques to define mesasand etch down to the n-type layer as shown in FIG. 16. A 1% bromine inmethanol solution is a suitable etchant for the Hg-based layers. Asolution of 1 gm potassium dichromate (K₂ Cr₂ O₇), 10 ml sulfuric acid(H₂ SO₄) and 20 ml of deionized water is suitable for etching ZnSe andrelated alloys. Both solutions are compatible with availablephotoresists. Following this etching step, ohmic contact to the n-typelayer can then be achieved by photolithographically defining and openingvia regions to the n-type layer and depositing an appropriate metal suchas indium, or indium/gold. Standard liftoff techniques can then be usedto remove the deposited metal from unwanted areas of the structure.

Referring now to FIG. 22, a high efficiency blue/green light emittingdiode according to the present invention is illustrated. Diode 10includes the quaternary alloy Zinc sulfur telluride selenide (ZnS_(x)Te_(y) Se_(z), where x+y+z=1) between layers of zinc sulfur selenide(ZnSSe). In particular, as shown in FIG. 22, a substrate 15 Of n=typegallium arsenide includes a layer of n-type zinc sulfur selenidethereon. Preferably, layer 16 is ZnS₀.07 Se₀.93, which is latticematched to gallium arsenide. Region 31 of ZnS_(x) Te_(y) Se_(z) isformed on region 16 and p-type zinc sulfur selenide region 17 is formedthereon. An ohmic contact such as a layer of zinc mercury selenide orzinc telluride selenide 1 and a layer of mercury selenide is included.Metal contact layers 13 and 14 are also included as was described above.Cladding layers 16 and 17 are preferably doped at 1×10¹⁸ dopants cm⁻²and layer 31 is preferably about 1000 Å thick for an LED and about100-200 thick for a laser. It will also be understood by those havingskill in the art that a multiple quantum well of zinc sulfur tellurideselenide and zinc sulfur selenide may be provided between claddinglayers 16 and 17.

FIG. 23 illustrates a similar structure which is optimized for formationon n-type zinc selenide substrate 15. Since the substrate 15 is zincselenide, sulfur is not employed in this structure.

Tellurium is known to form an isoelectronic trap in zinc selenide andzinc sulfur selenide which produces a broad and intense light output inthe blue/green spectral region as measured by photoluminescence andelectroluminescence experiments. The addition of tellurium to the activeregion 31 of the optical emitter produces an isoelectronic trap similarto that which occurs when tellurium is added to zinc selenide, whichresults in intense broadband photoluminescent and electroluminescentoutput in the green to blue/green spectral region at room temperature,depending on the tellurium content of the alloy 31. FIG. 4 illustratesthe spectral output (wavelength versus arbitrary intensity) of an LEDfabricated according to FIG. 22. At a temperature of 300° K. and aninput current of 50 milliamps, the green LED exhibits an absoluteefficiency η of 6×10⁻⁴ and an output power of 85 microwatts. The peakwavelength is 504 nanometers and the dominant wavelength is 503nanometers. The optical purity is 61%. Optimization of the materialsgrowth parameters, layer thicknesses and device packaging may beexpected to give rise to even higher LED efficiencies at roomtemperature.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

That which is claimed:
 1. An ohmic contact for a semiconductor deviceformed of Group II-VI compound semiconductor materials and including alayer of at least one of p-type zinc selenide (ZnSe) and an alloythereof, said ohmic contact comprising:a zinc telluride selenide(ZnTe_(x) Se_(1-x)) layer on said layer of at least one of p-type ZnSeand an alloy thereof, where 0<x<1; a mercury selenide (HgSe) layer onsaid ZnTe_(x) Se_(1-x) layer; and a conductor layer on said HgSe layer.2. The ohmic contact of claim 1 wherein x is constant in said ZnTe_(x)Se_(1-x) layer to provide a step graded ZnTe_(x) Se_(1-x) layer.
 3. Theohmic contact of claim 1 wherein x decreases across said ZnTe_(x)Se_(1-x) layer, from said ZnSe layer to said HgSe layer, to provide agraded ZnTe_(x) Se_(1-x) layer.
 4. The ohmic contact of claim 3 whereinx decreases linearly across said ZnTe_(x) Se_(1-x) layer, from said ZnSelayer to said HgSe layer, to provide a linear graded ZnTe_(x) Se_(1-x)layer.
 5. The ohmic contact of claim 3 wherein x decreases nonlinearlyacross said ZnTe_(x) Se_(1-x) layer, from said ZnSe layer to said HgSelayer, to provide a nonlinear graded ZnTe_(x) Se_(1-x) layer.
 6. Theohmic contact of claim 5 wherein x decreases parabolically across saidZnTe_(x) Se_(1-x) layer, from said ZnSe layer to said HgSe layer, toprovide a parabolic graded ZnTe_(x) Se_(1-x) layer.
 7. The ohmic contactof claim 6 wherein a first half of said ZnTe_(x) Se_(1-x) layer adjacentsaid ZnSe is p-type, and a second half of said ZnTe_(x) Se_(1-x) layeradjacent said HgSe layer is of neutral conductivity.
 8. The ohmiccontact of claim 1 wherein at least a portion of said ZnTe_(x) Se_(1-x)layer is p-type.
 9. The ohmic contact of claim 8 wherein said ZnTe_(x)Se_(1-x) layer is p-type adjacent said ZnSe layer, and is of neutralconductivity adjacent said HgSe layer.
 10. The ohmic contact of claim 1wherein said zinc telluride selenide layer is p-type modulation doped.11. The ohmic contact of claim 1 wherein said ZnTe_(x) Se_(1-x) layerincludes a thin layer of ZnTe adjacent said HgSe layer.
 12. Asemiconductor device formed of Group II-VI compound semiconductormaterials comprising:an optical emission heterostructure formed of GroupII-VI compound semiconductor materials, and including a layer of atleast one of p-type zinc selenide (ZnSe) and an alloy thereof; a zinctelluride selenide (ZnTe_(x) Se_(1-x)) layer on said layer of at leastone of p-type ZnSe and an alloy thereof, where 0<x<1; a mercury selenide(HgSe) layer on said ZnTe_(x) Se_(1-x) layer; and a conductor layer onsaid HgSe layer.
 13. The semiconductor device of claim 12 wherein x isconstant in said ZnTe_(x) Se_(1-x) layer to provide a step gradedZnTe_(x) Se_(1-x) layer.
 14. The semiconductor device of claim 12wherein x decreases across said ZnTe_(x) Se_(1-x) layer, from said ZnSelayer to said HgSe layer, to provide a graded ZnTe_(x) Se_(1-x) layer.15. The semiconductor device of claim 14 wherein x decreases linearlyacross said ZnTe_(x) Se_(1-x) layer, from said ZnSe layer to said HgSelayer, to provide a linear graded ZnTe_(x) Se_(1-x) layer.
 16. Thesemiconductor device of claim 14 wherein x decreases nonlinearly acrosssaid ZnTe_(x) Se_(1-x) layer, from said ZnSe layer to said HgSe layer,to provide a nonlinear graded ZnTe_(x) Se_(1-x) layer.
 17. Thesemiconductor device of claim 16 wherein x decreases parabolicallyacross said ZnTe_(x) Se_(1-x) layer, from said ZnSe layer to said HgSelayer, to provide a parabolic graded ZnTe_(x) Se_(1-x) layer.
 18. Thesemiconductor device of claim 17 wherein a first half of said ZnTe_(x)Se_(1-x) layer adjacent said ZnSe layer is p-type, and a second half ofsaid ZnTe_(x) Se_(1-x) layer adjacent said HgSe layer is of neutralconductivity.
 19. The semiconductor device of claim 12 wherein at leasta portion of said ZnTe_(x) Se_(1-x) layer is p-type.
 20. Thesemiconductor device of claim 19 wherein said ZnTe_(x) Se_(1-x) layer isp-type adjacent said ZnSe layer, and is of neutral conductivity adjacentsaid HgSe layer.
 21. The semiconductor device of claim 12 wherein saidzinc mercury selenide layer is p-type modulation doped.
 22. Thesemiconductor device of claim 12 wherein said ZnTe_(x) Se_(1-x) layerincludes a thin layer of ZnTe adjacent said HgSe layer.
 23. Asemiconductor device formed of Group II-VI compound semiconductormaterials comprising:an optical emission heterostructure formed of GroupII-VI compound semiconductor materials, and including a layer of atleast one of p-type zinc selenide (ZnSe) and an alloy thereof; at leastone of a zinc mercury selenide (Zn_(x) Hg_(1-x) Se) layer and a zinctelluride selenide (ZnTe_(x) Se_(1-x)) layer on said layer of at leastone of p-type ZnSe and an alloy thereof, where 0<x<1; a mercury selenide(HgSe) layer on said at least one of a Zn_(x) Hg_(1-x) Se and a ZnTe_(x)Se_(1-x) layer, said contact to said at least one of a Zn_(x) Hg_(1-x)Se and a ZnTe_(x) Se_(1-x) layer, and being sufficiently thin to passoptical emissions from said optical emission heterostructuretherethrough; and a conductor layer on said HgSe layer.
 24. Thesemiconductor device of claim 23 when said HgSe layer is less than about100 Å thick.
 25. The semiconductor device of claim 23 wherein saidconductor layer is a transparent conductor layer.
 26. The semiconductordevice of claim 25 wherein said conductor layer is indium tin oxide. 27.A semiconductor device formed of Group II-VI compound semiconductormaterials, comprising:an optical emission heterostructure formed ofGroup II-VI compound semiconductor materials, and including a layer ofat least one of p-type zinc selenide (ZnSe) and an alloy thereof; amercury selenide (HgSe) layer on said layer of at least one of p-typeZnSe and an alloy thereof, said HgSe layer being sufficiently thick toprovide an ohmic contact to said layer of at least one of p-type zincselenide layer and an alloy thereof and being sufficiently thin to passoptical emissions from said optical emission heterostructuretherethrough; and a conductor layer on said HgSe layer.
 28. Thesemiconductor device of claim 27 when said HgSe layer is less than about100 Å thick.
 29. The semiconductor device of claim 27 wherein saidconductor layer is a transparent conductor layer.
 30. The semiconductordevice of claim 29 wherein said conductor layer is indium tin oxide.